1. Field of the Invention
This invention relates to Doppler ultrasound medical systems and more particularly to the dealiasing Doppler audio output signals for improved sound quality in the audio output of the sampled data.
2. Description of the Related Art
In the medical industry, ultrasound systems have gained popularity for medical diagnosis because they are non-invasive and are capable of providing a detailed and accurate imaging of components of the body. Real-time Doppler ultrasound systems are used to image motion such as blood flow through arteries or the heart.
The Doppler effect is a physical phenomenon present when the source of a wave or signal is moving relative to its observer. The frequency (or inversely, the wavelength) of the signal increases or decreases depending upon the direction of motion. Doppler ultrasound signals can be transmitted as a "continuous" wave or a "pulsed" signal. By measuring the frequency of a sound signal which echoes off blood moving through an artery, for example, the velocity of the blood can be estimated.
Doppler ultrasound systems typically include two forms of signal output. A scrolling spectrogram image provides visual output while sound through stereo speakers provides audio output. Doppler ultrasound can determine the presence or absence of flow, the direction and speed of flow, and the character of flow.
In spectral Doppler ultrasound, the Doppler-shifted echoes coming from ultrasound fired into the body are received in analog form and are captured by the system's transducer. FIG. 1 shows a Doppler ultrasound system 10 which includes a transducer 11 for receiving an electrical pulse transmitting an ultrasonic pulse 12 into a reflection zone such as a region including a blood vessel 13. The reflection zone reflects sound echoes 14 back to the transducer 11.
Beamformer 15 controls the firing of the transducer 11 as well as delaying and summing the sound echo signals 14 received by transducer 11. The summed radio-frequency signal (RF) is then transmitted to a complex modulator and lowpass filter represented by box 16 to create the complex "baseband" signals that are conventionally referred to as the in-phase (I) and quadrature (Q) signals or "I/Q" signals 17. The I/Q signals then pass through a sampler, sometimes called a "range gate" 18.
The output of the sampler is processed for spectral display. The sampler output is also separated into forward and reverse components which are fed to a pair of stereo speakers. The sampler introduces an artifact called "aliasing" which is explained with reference to FIGS. 2 and 3.
FIG. 2 represents the frequency spectrum of a Doppler I/Q signal before sampling (the absolute value of the Fourier transform is on the y-axis and the frequency is on the x-axis). The positive portion of the frequency spectrum is to the right of DC and the negative portion is to the left.
FIG. 3, on the other hand, shows the signal of FIG. 2 where the sampling process of the ultrasound system of FIG. 1 has aliased the signal. The resultant aliasing of the signal in FIG. 2 as shown in FIG. 3 is described as follows: Any components in the I/Q input signal 17 with frequencies above half of the sampling frequency are "aliased." In FIG. 3, components above +F.sub.s /2 have wrapped around to negative frequencies. To the spectral display processor and the audio forward/reverse split, these aliased positive frequencies will appear negative, i.e. without correction, with the result that flow towards the transducer will be mapped as flow away from the transducer.
Again referring to FIG. 1, in the case of a spectrogram image on display 20a received from processor 20b, if the spectral image is aliased, the operator viewing a display screen can manually rotate or shift the spectral display by adjuster 21 so that the aliased frequencies are displayed correctly. Alternatively, the image can be rotated or shifted automatically. In either case, in practice, the preferred scanning mode is to scan with heavy aliasing, even as high as 100%. In such a case if the reverse flow is not of concern, the baseline can be adjusted to devote the entire range of velocity detection to forward flow. This technique doubles the maximum velocity that can be measured but leaves a very bad, aliased audio signal.
Therefore, it is highly desirable to duplicate the frequency spectrum of FIG. 2 before it is processed for audio output, that is, to provide a signal which is alias-free.
The above discussion provides the manner in which a particular type of ultrasound machine samples a signal. Other sampling configurations are also known. For example, in some machines the transducer elements control the delay and summing instead of the beamformer as discussed above. Moreover, this discussion is also intended to cover sampling of any type of Doppler processor.